I think you'll get black offspring in the first generation.
If you cross the daughters back to the father, you should get some black chicks and some black mottled chicks, about equal numbers of each.
If you cross the sons back to their mother, you should get some chocolate chicks and some black chicks, about equal numbers of each.
Or if breed the crossed males and females to each other, you should get black chicks, chocolate chicks, mottled chicks, and chocolate mottled chicks. Roosters would be about 3/4 black, 1/4 black mottled. Hens would be about 1/2 black and 1/2 chocolate, with 1/4 of each color of hens also having mottling.